Acceleration method and apparatus



United States Patent 3,375,396 ACCELERATION METHOD AND APPARATUS Jacob Haimson, Palo Alto, and Craig S. Nunan, Los Altos Hills, Calif. assignors to Varian Associates, Palo Alto. Calif., a corporation of California Filed Oct. 29, 1965, Ser. No. 505,630

4 Claims. (Cl. 3153.5)

The present invention is directed to an acceleration method and apparatus and more particularly to the linear acceleration of charged particles with a traveling radio frequency wave directed along the path of the charged particles.

One of the basic problems in the design and configuration of a linear particle accelerator for acceleration of charged particles such as, for example, electrons or positrons, is the establishment of a method and structure for propagating the charged particles along the linear path in proper interacting relationship with a radio frequency wave for effective capture of charged particles by the fields of the radio frequency wave and bun-ching and acceleration of the charged particles by the radio frequency wave as the particles and wave travel along together.

Typical traveling wave linear accelerating structures are in the form of disc loaded waveguides in which centrally .apertured discs are spaced periodically along a hollow cylindrical waveguide to define a series of axially aligned coupled cavities for passing a radio frequency electromagnetic wave in traveling wave interaction with a pulse of charged particles for acceleration of the charged particles to relativistic velocities. The spacing of the discs along the waveguide is selected so that the charged particles passing along the Waveguide are subjected to maximum accelerating fields existing within each cavity when the particle passes through the cavity. For this purpose accelerators are usually designed with the spacing L between the transverse midplanes of the cavities along the accelerating structure determined by the following formula:

Two techniques have been commonly adopted in the Q past for the initial capture and bunching of these injected low velocity particles. One method utilizes an accelerating structure having a uniform phase velocity of v equal to the velocity of light 0 with the pulse of charged particles arranged with respect to the pulse of the radio frequency wave so as to enter the waveguide during and prior to reversal of the electric field of the radio frequency wave. With this method the particles moving slower than the phase velocity of the wave move relative to the wave so as to efiectively slip back to the wave crest or maximum field strength of the wave after the particles have traversed a short distance and gained sufficient energy to become synchronous with the Wave. An

alternative technique makes use of an accelerating structure with a varied phase velocity so that in the initial portion of the accelerating structure the wave travels with a phase velocity less than the velocity of light and the wave phase velocity increases along the accelerating structure whereby the difference between the initial phase 3,3 75,396 Patented Mar. 26, 1968 velocity of the wave and the initial velocity of the injected particles is decreased from the diiference in a constant phase velocity structure. This technique permits the acceptance of particles that have been injected over a broader range of phase positions of the radio frequency wave than is normally possible with an accelerating structure having a constant phase velocity equal to the velocity of light.

The two above described techniques have been used to the present time reasonably effectively when utilizing high frequency accelerating structures such as operate with R-F waves in the S frequency band. In such structures where the spacing of the discs along the axis of the accelerator is such as to provide three or' four cavities per free space wavelength of the R-F wave the distances traveled by the charged particles in the cavity are short and there is a reasonably large degree of freedom in the injection of the particles for proper phasing with respect to the peak accelerating fields in the waveguide. However, in lower frequency accelerating structures, such as an L band, the distances traveled by the particle in each cavity becomes considerably greater and the proper phasing of the particles with respect to the peak fields of the waves becomes more critical. In fact, in some circumstances with lower frequency'structures, slower particles will be subjected to decelerating fields either at the input end of the first cavity or at the output end of the first cavity when injected to interact with strong accelerating fields during another portion of their passage through this first cavity. Thus, the desired accelerating'eifect of the maximum strength portion of the field may be lost or negated.

The object of the present invention is to provide a method and apparatus for accelerating charged particles to relativistic velocities wherein a desired phase relationship' between the injected particles and the maximum strength fields established by the accelerating radio frequency waves are maintained at the input end of the accelerating structure.

In accordance with the present invention the distance s from the location at which the charged particles of the particle beam are first subjected to the electromagnetic fields 'of substantial strength set up within the first cavity resonator by the radio frequency electromagnetic wave introduced therein to the nearest plane which is spaced the distance L'from the transverse mid-plane oi the second cavity resonator is determined by the formula:

With this construction, whether the accelerating structure is designed with a constant phase velocity v or a. variable phase velocity v the distance over which the particles are subjected to thefields of the radio frequency wave is reduced from the normal maximum full cavity length to a lower value which may be as short as onehalf a normal cavity length.

In accordance with one specific embodiment of the present invention, the first cavity resonator of the accelerating structure is provided with an input drift tube which shields the particles being introduced into the first cavity from the eifects of substantial electromagnetic fields within the cavity until the particles have traveled to a distance somewhere between the inner wall of the input disc and the mid-plane of the cavity.

One way of analyzing the wave effects on the charged particles in the first cavity resonator of the accelerating structure is to consider the field effects there as the typical effects of a standing wave in the first half of the first cavity and as the typical effects of a traveling wave from the mid-plane of the first cavity throughout the remainder of the accelerating structure. When the accelerating characteristics of the fields are thus viewed, it becomes desirable to reduce or take advantage of the effects of the standing wave fields on the particles.

Other objects and advantages of this invention will become apparent upon a perusal of the following specificat-ion taken in conjunction with the accompanying drawings wherein similar characters of reference represent corresponding parts in each of the several views.

In the drawings:

FIG. 1A is a schematic side elevational sectional view partially in block form of a conventional linear accelerating apparatus;

FIG. 1B is a graph of the relative electric field intensity (E plotted versus the axial distance (2) along the accelerating structure shown in FIG. 1A;

FIG. 1C is a graph of the phase shift plotted versus the axial distance along the accelerating structure shown in FIG.'1A and illustrating the phase velocity (v of an R-F wave traveling along the structure of FIG. 1A;

FIG. 2 is a side elevational sectional view illustrating one embodiment of the present invention; and

FIG. 3 is a side elevational sectional view illustrating another embodiment of the present invention.

Referring now to the drawings, there is shown in FIGS. 1A, lB and 1C a conventional linear particles accelerating apparatus and graphs illustrating certain characteristics of the apparatus. As shown in FIG. 1A, the particle accelerator includes a particle generating assembly A for directing charged particles such as, for example, electrons or pos-itrons, into the input end and through an accelerating structure 1B wherein a radio frequency wave introduced into the accelerating structure B via an input coupling assembly C propagates for interaction with an acceleration of the charged particles. :For the case of electrons, the particle generating assembly A can be any conventional assembly such as, for example, including a cathode for producing a beam of electrons and a chopping and/or prebunching cavity or beam deflection assembly for collecting all of the beam except time spaced pulses of charged particles which are passed through an orifice into the input end of the accelerating structure B.

The accelerating structure B includes a disc loaded waveguide having a hollow cylindrical side wall 11 and a plurality of .apertured discs 12 located at spaced apart positions longitudinally within the wall 11 and with the apertures therein substantially centered on the longitudinal axis of the wall 11. The side wall 11 and each pair of adjacent discs 12 define a cavity resonator D through which the pulse of charged particles is passed in interacting relationship with a radio frequency wave effectively traveling along the accelerating structure B by being coupled through the apertures in the discs 12 from cavity to cavity. I

The radio frequency wave is coupled into the input or first cavity resonator D from a rectangular waveguide 21 via an iris opening 22. Commonly, the waveguide 21 is rectangular in cross section operating in the TE mode and is iris matched to the first or input cavity D such that the first order radial mode is excited in cavity D by magnetic coupling and the desired TM accelerating mode is launched along the disc loaded waveguide circuit.

In order to maintain the desired phase relationship between the charged particles traveli-ng along the waveguide so that the particles pass through each cavity resonator in interacting relationship with the maximum accelerating field possible when the particles are to be directed along the entire structure, the longitudinal distance L from the mid-plane of one cavity to the mid-plane of the succeeding cavity or from the center plane of one disc to the center plane of the next disc is determined by the formula:

as N

where 0 equals v /c, A is the free space wavelength of the traveling electromagnetic wave, N is the number of discs axially of the waveguide per wavelength A and typically three or four in number, v is the plane velocity of the traveling electromagnetic wave along such waveguide and c is the velocity of light. 5,, is typically one. It is also typically equal to or approximately equal to fi where B is v /c and v is the velocity of the charged particles such as electrons.

In accordance with the present invention, in order to reduce the problems of properly phasing the charged particles with reference to the traveling radio frequency wave along the accelerating-structure so as to expose the charged particles 'to only desired fields when there 1s a large differential between the velocity of the partlcles and the phase velocity of the traveling radio frequency wave, the distance s between the location at which the particles of the particle beam are first subjected to substantial fields within the first cavity resonator and the nearest plane which is spaced a distance L from the transverse mid-plane of the second cavity resonator of the waveguide is determined by the formula:

As shown in FIG. 2 the accelerating waveguide B of the disc loaded variety having three discs 12' per wavelength A is provided with a hollow cylindrical drift tube 31 located about the aperture 10 in the input wall 23 of the input cavity D and extending axially inwardly of the first cavity resonator D to a position lying between the inner surface of input wall 23 and substantially the mid-plane of the cavity resonator D As shown in SOlld lines the inner end of the drift tube 31 lies between the inside surface of the input wall 23 of the first cavity D and therefore s has a finite value less than fl X/ZN. Alternatively, as shown in phantom the drift tube 31 can extend to the cavity mid-plane in which case s equals 0.

In accordance with another embodiment of the present invention as illustrated in FIG. 3, the input coupler cavity D in the accelerating waveguide B" is shortened axially longitudinally of the accelerating B" thereby reducing the effective interaction length between the particles introduced into the first cavity resonator and substantial electromagnetic fields set up in the first cavity resonator by the radio frequency wave introduced thereinto through the input waveguide 21". Here again, as in FIG. 2, the structure shown in solid form illustrates a case in which the distance s is greater than 0 but less than B R/2N while the structure shown in phantom illustrates a structure in which the distance s equals 0. As is illustrated in FIGS. 1A and 1B, electromagnetic fields of low intensity actually extend partially down the particle input orifice 10. Therefore, in defining s reference is made to the subject of the particles to substantial electromagnetic fields.

One way of analyzing the advantage of the present invention and the operative characteristics of a traveling wave linear accelerator is by a theory which analyzes the fields set up in the input or first cavity resonator as typifying the fields of a standing wave over the distance s from the location of the injection of the charged particles to be accelerated to the nearest plane spaced a distance L from the transverse-mid-plane of the second cavity resonator.

This theory is applicable to the results of tests made to determine the relative electric field intensity E and the total phase 0 measured on an actual prior art traveling wave linear accelerator waveguide as illustrated in FIG. 1A. The test results are illustrated in FIGS. 1B and 1C,

respectively. The values of FIGS. 1B and 1C were measured by perturbation experiments conducted on a short length of s band uniform impedance accelerating waveguide constructed having a phase shift of 211'-/ 3 per cavity resonator and with the following dimensions: L= 1.3772", 2a=0.8007", 2b=3.277l" and t=0.2300".

For these experiments the waveguide B was fitted with input and output symmetrical field iris couplers such as assembly C with the output coupler terminated in a matched load to establish operational traveling wave conditions, A small dielectric bead supported on a taut nylon thread was moved along the axis of the accelerating structure B while the reflection coefiicient (p) and the phase of the reflection were recorded against distance (z). Since the electrical field intensity (E is proportional to the square root of the reflection coefiicient the maximum amplitude over the R-F cycle of the total electrical field (E at any point z was calculated and plotted from the information obtained.

The measurements indicate that the field intensity (E increases approximately sinusoidally from a cut-01f point (2 outside the first cavity resonator to the first maximum (E located at the transverse mid-plane of the input cavity D Thereafter the plot of field intensity (E as shown in FIG. 1B exhibits a periodic pattern with maxirna (E at the cavity midplanes (2:0, L, 2L, etc.) and minima of approximately 40% E at the mid-plane of The total field phase plot (0 as plotted in FIG. 10 can be interpreted in terms of a traveling wave having a fluctuating velocity of propagation with a 120 periodic oscillation about the waveguide mean phase velocity straight line which for this case is a phase velocity v equal to the velocity of light c. Thus the phase divergence between the O curve for the total field phase plot and the straight line for v equal to c would indicate that in the central region of the cavities the propagation velocity v is greater than the velocity of light 0. Similarly, in the vicinity of the discs 12 the velocity would be considerably less than c, and that at the parallel tangent points where z is 3L/ 8 and 5L/ 8, v would equal c.

The loss of the periodicity of the H curve from the expected plot indicated in FIG. 1C in phantom from X to 0 to the determined plot shown from Y to 0 over the first half of the input coupled cavity D can be interpreted as indicating the presence of an infinite velocity wave in the first half of this cavity and would corroborate the existence of a standing wave domain in this region. The small reverse phase shift shown in this cavity from point Y to point 0 differing from a truly horizontal line would indicate that the coupler fields were not perfectly matched as would have been the case for a constant 1%.

This standing wave field pattern in the first half of the input coupler cavity D results in an error in the employment of conventional traveling wave concepts for beam analysis in the initial half of the input coupler cavity D This error between an actual standing wave domain and an assumed traveling field domain will produce a phase error of 60 for a velocity of light particle that is considered to enter on the crest of the field. Thus, for a 120 long cavity a velocity of light particle must be injected 60 earlier to become phased at the crest of the traveling wave, Such early injection would result in deceleration of the particles. Especially in the case of intense .beam currents it would result in serious debunching due to space charge forces being more eflFective while the particles are at reduced energy due to such deceleration.

Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, it is understood that certain changes and modifications may be practiced within the spirit of the invention as limited only by the scope of the appended claims.

What is claimed is:

1. A traveling wave linear accelerator comprising, in combination: a disc loaded waveguide including a plurality of cavity resonators arranged for passage of a particle beam from an input end of said waveguide therethrough to an output end for interaction with and acceleration by a traveling radio frequency electromagnetic wave propagating therethrough, each of said cavity resonators defined by a hollow cylindrical wall and apertured input and output discs closing the opposite ends of said wall, the apertures in said discs centered substantially axially of said waveguide for passing the particle beam through each of said cavity resonators in said waveguide and coupling electromagnetic energy between cavity resonators for propagating said traveling radio frequency electromagnetic wave along said waveguide, substantially every cavity beyond the first cavity resonator at the input end of said waveguide proportioned so that the distance L from the cavity transverse mid-plane to the transverse mid-plane of an adjacent cavity resonator is determined by the formula:

where [3,, is v /c, A is the free space wavelength of the traveling electromagnetic wave, N is the number of said discs axially of said waveguide per wave length x, v is the phase velocity of the traveling electromagnetic wave along such Waveguide and c is the velocity of light; means for coupling a radio frequency electromagnetic wave into said first cavity resonator for setting up substantial electromagnetic fields in said first cavity resonator and establishing a traveling radio frequency electromagnetic wave propagating along said waveguide; and means for directing the particle beam into said first cavity resonator for passage along said waveguide, said first cavity resonator constructed such that the distance s from the location at which the particles of said particle beam are first subjected to said fields within said first cavity resonator to the nearest plane which is spaced the distance L from the transverse mid-plane of the second cavity resonator of said waveguide is determined by the formula:

2. The traveling wave linear accelerator in accordance with claim 1 characterized further in that said input cavity resonator includes a hollow cylindrical drift tube located about the aperture in the input disc of said first cavity resonator and extending axially inwardly of said cavity resonator with the inner end of said drift tube defining said location at which said particles are first subjected to said fields within said first cavity resonator.

3. The traveling wave linear accelerator in accordance with claim 1 characterized further in that said location at which said particles are first subjected to said fields of said traveling radio frequency electromagnetic wave lies at the aperture in the input disc of said first cavity resonator.

4. The method of accelerating charged particles to relativistic velocities comprising the steps of: establishing a wave-particle interaction structure by arranging a plurality of apertured discs along a linear path and closing the region between the periphery of adjacent discs to define a series of coupled cavity resonators along said linear path for passing a particle beam in interacting relationship with a traveling radio frequency electromagnetic wave with the length L between the transverse mid-planes of substantially every pair of adjacent cavity resonators determined by the formula:

where B is v /c, x is the free space wavelength of such electromagnetic wave, N is the number of such discs along said interaction path per wavelength A, v,, is the phase velocity of such electromagnetic wave along such interaction path and c is the velocity of light; coupling a radio frequency electromagnetic wave onto said interaction path between a pair of such discs to set up electromagnetic fields along said path for establishing a traveling radio frequency electromagnetic wave propagating along said interaction path; generating a beam of charged particles and directing such beam along said interaction path for interaction with such traveling electromagnetic wave with the charged particles in said beam first subjected to electromagnetic fields along said path at a location between said pair of discs and at a distance s from the nearest plane lying at a distance L from the transverse mid-plane between the next pair of discs along such path in the moving direction of particles and where s is determined by the formula:

BM 05K 2N No references cited.

10 HERMAN KARL SAALBACH, Primary Examiner.

S. CHATMON, JR., Assistant Examiner. 

1. A TRAVELING WAVE LINEAR ACCELERATOR COMPRISING, IN COMBINATION: A DISC LOADED WAVEGUIDE INCLUDING A PLURALITY OF CAVITY RESONATORS ARRANGED FOR PASSAGE OF A PARTICLE BEAM FROM AN INPUT END OF SAID WAVEGUIDE THERETHROUGH TO AN OUTPUT END FOR INTERACTION WITH AND ACCELERATION BY A TRAVELING RADIO FREQUENCY ELECTROMAGNETIC WAVE PROPAGATING THERETHROUGH, EACH OF SAID CAVITY RESONATORS DEFINED BY A HOLLOW CYLINDRICAL WALL AND APERTURED INPUT AND OUTPUT DISC CLOSING THE OPPOSITE ENDS OF SAID WALL, THE APERTURES IN SAID DISC CENTERED SUBSTANTIALLY AXIALLY OF SAID WAVEGUIDE FOR PASSING THE PARTICLE BEAM THROUGH EACH OF SAID CAVITY RESONATORS IN SAID WAVEGUIDE AND COUPLING ELECTROMAGNETIC ENERGY BETWEEN CAVITY RESONATORS FOR PROPAGATING SAID TRAVELING RADIO FRQUENCY ELECTROMAGNETIC WAVE ALONG SAID WAVEGUIDE, SUBSTANTIALLY EVERY CAVITY BEYOND THE FIRST CAVITY RESONATOR AT THE INPUT END OF SAID WAVEGUIDE PROPORTIONED SO THAT THE DISTANCE L FROM THE CAVITY TRANSVERSE MID-PLANE TO THE TRANSVERSE MID-PLANE OF AN ADJACENT CAVITY RESONATOR IS DETERMINED BY THE FORMULA: 